The present invention relates to target structures for television-camera pick-up tubes.
A well known problem in pick-up tube target structures concerns the minimization of dark current. The side of the target structure facing the scanning electron beam is charged by the latter to cathode potential. When image light is incident upon the other side of the target structure, the optically liberated charge carriers effect a discharge of the accumulated negative charge on the beam-side of the target, to an extent and with a geometry dependent upon the intensity and spatial composition of the image light. In the absence of incident light, it is of course necessary that the accumulated negative charge on the beam-side of the target structure, deposited there by the scanning beam, not be permitted to discharge. If the discharge of the accumulated negative charge occurs in the absence of incident light, then the resultant flow of dark current incorrectly simulates the incidence of non-existent scene light. Therefore, in the prior art, various expedients are known for ensuring that the discharge of the negative charge deposited on the target by the scanning electron beam occurs in dependence upon the number of charge carriers optically liberated by incident scene light, and not as a result of simple conductive current flow of beam-deposited negative charge carriers to the signal electrode of the target structure.
The most common of these prior-art techniques for minimizing dark current involve the use of p-n or blocking-layer junction structures. Thus, for example, in the case of plumbicon tubes, there is deposited on the front glass plate of the structure a transparent signal electrode, and deposited on the transparent signal electrode is a layer of n-type PbO followed by a layer of p-type PbO. The p-n PbO target structure acts as a diode, and is maintained reverse-biased by the operating voltage of the pickup tube. When the scanning electron beam deposits negative charge carriers on the beam-side of the p-n structure, these negative carriers cannot cross the reverse-biased p-n junction, except to a very small extent corresponding to the reverse-bias current flow through the diode structure; accordingly, dark current (discharge of the accumulated negative charge in the absence of incident scene light) is minimized. In contrast, when scene light is incident upon the target structure, the charge carriers optically liberated within the photoconductive material of the target can cross the reverse-biased p-n junction (i.e., from the n to the p side thereof) and effect dicharge of the beam-deposited electrons.
The establishment of such a p-n diode junction in the PbO of a pumbicon target structure involves only the appropriate p- and n-doping of the PbO, inasmuch as PbO is amphoteric. Where the photoconductive material of the target structure is not amphoteric, the requisite p-n diode junction is formed from two different materials, i.e., a heterocontact. For example, CdSe is a most preferred photoconductor for pick-up tube target structures, because of its excellent spectral response and sensitivity and because it is not damaged by exposure to very bright scene light. However, CdSe is not amphoteric, and can only be doped to be of n-type, not p-type. Therefore, in order to create the p-n junction requisite for minimization of dark current, the n-type CdSe layer of the target is covered by a layer of p-type material, such as p-type CdTe, or the like. The resultant p-n heterocontact minimizes dark current in substantially the same way as the p-n layer of amphoteric PbO in a plumbicon tube.
The use of p-n junctions does minimize dark current, and is therefore successful in that sense. However, pick-up tube target structures using p-n junctions inherently tend to have a lower sensitivity than, for example, comparable target structures making use of essentially a single homogeneous layer of CdSe. The amplification which can be achieved using known p-n junction target structures is generally considerably less than unity, and greater-than-unity amplification (i.e., true gain) has been heretofore impossible. The maximum theoretical amplification in p-n junction target structures is limited to unity. I.e., if each incident scene light quant is absorbed by the photoconductive layer of the target structure and optically generates one electron-hole pair, and if the optical generation of each electron-hole pair results in the discharge of one beam-deposited electron on the beam-side of the target structure, then each incident light quant discharges one beam-deposited electron, and the amplification would be unity. Unity amplification has been conceived of as the inherent limit of amplification in p-n junction target structures of the type in question, because the mechanism which the prior art establishes in such target structures for the discharge of beam-deposited electrons is based upon discharge by primary photocurrent, i.e., discharge by recombination with optically liberated charge carriers. Of course, in reality, the value of the amplification in p-n junction target structures is considerably less than unity.
Greater-than-unity amplification (gain) would result if, somehow, each incident light quant, and therefore each electron-hole pair generated by the incident light quant, could effect the removal of more than one of the beam-deposited electrons accumulated at the beam-side of the target structure.
Greater-than-unity amplification (gain) can be achieved in, for example, vidicon target structures not employing a p-n junction. In such a vidicon target structure, of each electron-hole pair generated by an absorbed light quant, the hole is trapped within the photoconductive layer of the target, whereas the electron quickly travels to the anode. The resultant loss of electrical neutrality must be compensated by introduction of replenishing electrons into the photoconductive layer. However, the beam-deposited electrons accumulated on the beam-side of the target structure cannot themselves effect this replenishment and restoration of electrical neutrality. Instead, restoration of electrical neutrality is effected by electrons coming directly out of the scanning beam itself. Thus, reestablishment of electrical neutrality within any single picture element of the target structure can only be achieved during the short time interval that the scanning electron beam is actually incident upon this picture element to inject electrons which recombine with the trapped holes. Thus, two counteracting effects are involved: one, the continual generation of such trapped holes during scene light incidence; and two, the injection of electrons directly from the scanning beam which then recombine with the trapped holes, to restore electrical neutrality. The equilibrium between these two processes is established only slowly. During operation, there may at any one time be a greater number of thusly trapped optically generated holes than can be generated during a single image period. As a result, during a single image period, the number of beam-injected electrons introduced into the photoconductive layer may be greater than the number of holes optically generated during that particular image period. This is often referred to in the art as "stack gain". In this situation, the reproduced image derived from the T.V. camera tube will form only gradually and likewise will disappear only gradually. Of course, this sluggishness of response is completely unacceptable for most practical applications, even though in a certain sense greater-than-unity amplification (gain), i.e., within certain image periods, may have been acheived.
Greater-than-unity amplification (gain) can also be achieved in CdSe photocells, if they are provided with gold blocking contacts. In this case, the electrodes form Schottky junctions, as a result of the high work function of the gold. Diffusion of electrons into the CdSe is prevented by a potential barrier which cannot be overcome by thermal excitation alone. However, its effect is reduced with increasing field strength due to tunneling effects resulting in an increase in current, expoentially dependent upon the square root of the applied voltage. Additionally, when light is incident, there is a further increase in charge carrier injection at the junction, caused by an increase in the (positive) space charge density in the surface layer of the photoconductor, even at low levels of illumination. Although greater-than-unity amplification (gain) is achieved in this way, the use of gold blocking junctions is not applicable to camera tube target structures, because the use of metal layers at the beam-side of the target is impermissible. The layer upon which the beam-deposited electrons accumulate must be of high resistivity (greater than 10.sup.8 ohm-centimeters), in order to prevent the loss of image resolution which would occur if the non-uniform accumulation of beam-deposited electrons on the beam-side of the target could equalize itself by electron travel in the direction parallel to the target surface. Accordingly, the gold blocking layers referred to above cannot be used.
Finally, it is known in the art to increase the sensitivity of a p-n junction target structure (wherein for example the n-layer is CdSe and the p-layer is Se mixed with As and Te) by subjecting the n-layer to oxidation prior to deposition of the p-layer thereon. A meaningful increase of sensitivity results, for reasons which hitherto have not been understood, but in any event greater-than-unity amplification (gain) is not even approached.